[olympus]

You need a system with more than two planets. A system with only two planets is going to be linearly spaced on any type of scale.

Once you've filtered out systems with two or fewer planets, you need orbital measurements with decent precision to tell whether a plot of planet spacing is actually linear on a log scale or not. Measuring a planet's orbit requires a decent amount of observation (since we can't measure star/planet mass directly and don't usually measure period directly), so unconfirmed systems likely don't have good measurements.

It's likely that most of the unmentioned systems get filtered out by one of the above two criteria.

[grkvlt]

Also, even if we detect a system with N > 3 exo-planets, we often cannot tell if there are more than N, but below the threshold of detectability, making the plots incomplete and incorrect.